A combustion temperature of a gas generating agent should be decreased in order to obtain an inflator for a vehicle airbag having a reduced size and weight. The combustion temperature can be decreased by adding a cooling agent (an additive for decreasing the combustion temperature), but this sometimes results in increase of the generated amount of mist (solid components discharged when the inflator is actuated).
JP-A 9-165287 discloses a gas generating composition including iron oxide as a cooling agent, wherein 50% by mass or more of the iron oxide have a mean particle diameter of larger than 100 μm.
JP-A 2004-155645 discloses a gas generating composition including aluminum hydroxide as a component for decreasing the combustion temperature and also improving ignition ability. It is described that the average particle diameter (D50) of aluminum hydroxide is preferably 0.1 to 70 μm, more preferably 0.5 to 50 μm, even more preferably 2 to 30 μm, but a particle size distribution is not described.